The present invention relates to multi-layer stretch cling films having little or no cling or blocking in an unstretched state and comparatively little or no unwind noise as the film is removed from a stretch wrap film roll, but having acceptable cling performance when in a stretched state.
In commercial packaging art, individual articles as well as bundles of articles, including bundles of articles on pallets, are frequently packaged using so called stretch cling film in a stretch wrapping technique. In the stretch wrapping technique, a thin web of film is stretched and applied under tension around the articles to be wrapped or bundled. After wrapping is complete, the film attempts to relax and thus applies a compressive force on the bundled articles prohibiting movement. It is desired that the stretch film have some inherent cling such that it will adhere to itself, and prevent relaxation back to its unstretched state so that the film will not unwrap and cause slippage of the stretched overlaid film layers. Two methods commonly used to give stretch wrap film inherent cling are the addition of high hexane extractables and the addition of low molecular weight olefinic additives.
The use of high (such as greater than 3.5%, for example) hexane extractables in the outermost layers of polyethylene film is described in U.S. Pat. Nos. 5,617,707; 5,516,475; 5,334,428; and 5,273,809; all of which are incorporated herein by reference. A drawback of using high hexane extractables containing polymers in the outermost layers exist in that high hexane extractables are believed to contribute to the problem of die lip build-up during extrusion and a build-up of low molecular weight olefinic material on fabrication equipment. Die lip build-up is typically degraded low molecular weight polyethylene and is undesirable because build up on the die lip can cause die lines that are inherent weak spots in film. Die lip build-up can also be pulled from the die as the film is extruded, resulting in black or brown spots in the film that can cause premature failure in film performance and are aesthetically unpleasing. A build-up of low molecular weight olefinic material is undesirable because the film surface may be negatively affected as the film is pulled through the fabrication equipment and may result in inconsistent cling performance.
Another concern with higher levels of hexane extractables is the contribution they make to film blocking on the roll. Films that contain resins with high levels of hexane extractables in the outermost layers can block while on the roll making the film difficult to remove resulting in holes and tears in the film prior to application. Therefore, in preferred embodiments of the present invention, the stretch wrap films have a hexane extractable level of less than about 3.5% by weight. In another preferred embodiment of the invention, the hexane extractable level is less then approximately 2% by weight in the resin or resins used in the skin layer. Also, in another preferred embodiment of the invention, no hexane extractables are used in the skin layer.
Using a tackifying or cling agent such as polybutene, polyisobutylene, and the like are also known in the art as disclosed in U.S. Pat. Nos. 5,569,693; 5,212,001; 5,175,049; 5,173,343; 5,154,981; 5,147,709; 5,141,809; 5,114,763; and 5,112,674; all of which are incorporated herein by reference. Using a cling agent adds to the cost of the final product, requires either preblending or alterations to production equipment to incorporate, and requires aging to bloom to the surface, all of which are negative aspects of its use.
It has also been disclosed that copolymers of ethylene and functional copolymers such as acrylates and vinyl acetate may be used as cling agents. For example, see U.S. Pat. Nos. 5,212,001; 5,173,343; 5,154,981; 5,141,809; 5,112,674; and 5,049,423; all of which are incorporated herein by reference. The copolymers of mention tend to be more expensive than conventional low density polyethylenes (LDPE) and linear low density polyethylenes (LLDPE) commonly used in stretch cling film formulations and tend to have lower ultimate enlongations than LLDPE resins, all of which are negative aspects for their use. Thus, in a preferred embodiment of the present invention, no cling additives are used in the films of the present invention.
As stated above, stretch wrap films are typically used in wrapping palleted loads of boxes, as well as individual boxes or articles such as furniture and the like. The stretch wrap films of the present invention may be applied using various techniques such as wrapping by hand or wrapped using a mechanical apparatus specially designed to wrap pallets, for example. In one procedure for using stretch wrap films of the present invention, the load to be wrapped is to be placed upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Rotational speeds are typically 5 to 50 revolutions per minute. Regardless of the specific wrapping procedure used, at the completion of the wrapping, the stretch wrap film is cut and attached to the underlying layer by cling, tack sealing, adhesive tape, spray adhesives, and the like. When the stretch film of the present invention is applied by hand, the operator typically holds an applicator or wrapping tool that holds the stretch wrap film roll. After the operator completes wrapping, the stretch wrap film is cut or torn.
A significant concern to many users of stretch cling film is the high noise levels associated with its use. For example, some automated stretch wrap application units may apply the stretch wrap film at very high rates (i.e., 50 to 200 feet per minute). When a stretch wrap film is applied at such high rates, it can create noise at very high levels (i.e., 80 to 100 dB). While not necessarily uncomfortable to all users, prolonged and repeated exposure to such noise levels without protective equipment can lead to discomfort and well as temporary or permanent hearing loss.
There have been attempts to create stretch wrap film having reduced unwind noise that include the use of low molecular weight additives. See, for example, U.S. Pat. Nos. 5,569,693 and 5,147,709, both of which are incorporated by reference. Research Disclosure 3771, published anonymously, incorporated herein by reference, discloses that altering existing stretch wrap film application equipment can reduce the noise level of film applications. The stretch wrap film of the present invention reduces unwind noise without the addition of such additives or altering application equipment.
The present invention is a novel multi-layer stretch wrap film comprising at least 3 layers and having minimal cling or blocking when in an unstretched state (such as when wrapped on a stretch wrap film roll) and having substantial cling when elongated (such as when stretched around palleted loads). The stretch wrap films of the present invention also have excellent mechanical properties and stretch film performance. For films of the present invention, film noise when unwinding at greater than 50 feet/minute is preferably less than 80 dB, more preferably less than 70 dB. Most preferably the film noise when unwinding at 50 to 200 feet/min is less than 60 dB.
Mechanical properties that are important in stretch films include, for example, how far the film can be stretched (i.e., stretchability), the film""s resistance to stretching, film tensile strength, film puncture resistance, film tear resistance, wrap, clarity and cling force. The films of the present invention are excellent in the above features.
The stretch wrap film of the present invention comprises an outer, or skin layer that provides for the quiet unwind characteristics. The stretch wrap film also comprises a sub-skin layer that provides cling characteristics and a core layer that assists in providing mechanical strength.
Preferably the stretch wrap films of the present invention comprise five or seven layers.
The resins used in the film composition include polypropylene, ethylene-propylene copolymers, low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), metallocene-catalyzed polyethylene (mPE), very low density polyethylene (VLDPE), ultra low density polyethylene (ULDPE), and blend combinations thereof.
In accordance with the present invention, a film prepared with the described composition is especially useful in stretch wrapping, stretch bundling, and tension winding operations for wrapping or holding small or large goods.
Advantages of the disclosed invention include reduced die lip build-up, reduced accumulation of low molecular weight species on fabrication equipment, and low unwind noise. All are accomplished without added product cost or modification to existing fabrication or application equipment.
The films of the present invention may also be used in rewinding/slitting operations whereby the stretch wrap film is actually rewound in a secondary operation onto smaller cores or rewound and slit into smaller widths.